1. Field of Invention
Various methods have been used to separate mixtures in chemical and other industries. These are distillation, freezing [or fractional solidification], crystallization, extraction, absorption and adsorption processes. A freezing process has several unique advantages such as (1) a broad field of application, (2) a high degree separation in a single step operation, (3) insensitivity to corrosion, (4) minimal pretreatment required, and (5) low energy input required. However, the major disadvantage of a freezing process is that it is not easy to separate a mixture beyond the eutectic composition of the system. Professor A. J. Barduhn of the University of Syracuse has introduced a eutectic freezing process using a secondary refrigerant. In the process, a secondary refrigerant such as Freon 114 is brought in direct contact with a eutectic mixture to form two or more solid phases. It has been demonstrated that this process can be used in separating eutectic mixtures such as water-NaCl, and water-KCl. There are some disadvantages to this process, such as (1) low recovery, (2) small crystal sizes, (3) needs to recover the refrigerant from product streams, and (4) refrigerant loss.
The process introduced herein does not bring a secondary refrigerant into direct contact with a eutectic mixture. Rather, the volatile component or components or the feed are vaporized to cool the mixture and thereby form two or more solid phases from the mixture. Since the volatile component or components of the feed are used as the refrigerant it is called the Primary Refrigerant Eutectic Freezing Process [PREUF Process]. Most of the disadvantages of the conventional eutectic freezing process have been removed.
2. Brief Description of Prior Art
When a freezing process is used in separating a mixture, it is difficult to separate the mixture beyond the eutectic composition of the system. A eutectic freezing process is used to overcome this problem.
Eutectic freezing using a secondary refrigerant was first proposed by Professor A. J. Barduhn of Syracuse University and some of the important aspects were tested at the University. This work is described in the following references:
Reference No. 1:
Allen J. Barduhn, "Waste Water Renovation-Part 1. A Design Study of Freezing and Gas Hydrate Formation," AWTR-4, Environmental Health Series, U.S. Dept. HEW-Public Health Service, October 1963. PA0 Allen J. Barduhn and A. Manudhane: "Temperatures Required for Eutectic Freezing of Natural Waters" Published in Desalination, 28 (1979) 233-241. PA0 Allen J. Barduhn, "The Freezing Processes for Desalting Saline Water," XII, the Congress, Int. Inst. Refrigeration, Madrid, September, 1967. Proceedings 1, P2B pages 1-19 (1967). PA0 G. L. Stepakoff, D. Siegelman, R. Johnson and W. Gibson, Fourth Int. Symp. on Fresh Water From the Sea, Heidelberg, 3 (1973) 421-33.
Reference No. 2:
This process is referred to as the Secondary Refrigerant Eutectic Freezing Process [SREUF Process]. In contrast, the process of the present invention is referred to as the Primary Refrigerant Eutectic Freezing Process [PREUF Process].
The SREUF Process may be considered as a variation on the normal freezing process for desalting seawater or brackish waters and to understand the former, one has to be familiar with the latter. The normal freezing processes have been described in the following reference:
Reference No. 3:
The thermodynamics of eutectics have been explained by Barduhn in references 1 and 2 and by Stepakoff in the following reference:
Reference No. 4:
As long as heat removal occurs by direct contact heat transfer, the ice and salt crystals can be nucleated and grown as separate, distinct phases in a continuously stirred tank crystallizer. Due to the fact that the ice and salt crystals are not mechanically interlocked, as in the case of metallic eutectics, physical methods of separation are possible. Using the simple system NaCl-H.sub.2 O the essential feature is that at -21 degrees Celsius, both ice and NaCl-2H.sub.2 O crystals precipitate from solution as heat is removed. One may thus maintain these two solid products in a slurry using a recycled brine stream (23.3% NaCl) and remove heat from them to produce additional crystals. The slurry effluent can be separated into one stream containing brine plus ice and another of brine plus solid salt (sodium chloride di-hydrate NaCl.2H.sub.2 O) since ice floats and the salt sinks. There is no brine product.
These ideas are incorporated into a simple SREUF Process. The feed is precooled by exchange with products and heat is removed by evaporation of a refrigerant such as n-butane or R-114 (CClF.sub.3 -CClF.sub.3) in direct contact with the aqueous solution. The effluent from the eutectic freezer is separated into the two streams and the salt slurry is filtered to produce the wet solid salt product. The ice slurry is sent to a wash column to remove salt prior to melting. Further details are available on how a wash column works and the various methods of direct contact melting of ice in references 3 and 4.
The AVCO Corporation was interested in treating inorganic industrial waste and then developed the process successfully through the pilot plant stage. This work was reported in reference 4 given earlier. They found that the ice and salt crystals which are formed in the concentrated eutectic solution were rather small (125 microns and 40 microns respectively) and presumed that ice-brine separation would be difficult in a conventional wash column. They devised and successfully operated a modified separation column, called a floatation column, in which all the operations of ice separation, washing and melting occur.
The SREUF Process may be used in separating mixtures of binary eutectic, ternary eutectic, quaternary eutectic, etc. In processing a binary eutectic, two solid phases are formed; in processing a ternary eutectic, three solid phases are formed, etc.